The principles of the construction of high-pressure sodium discharge lamps are known. It has also been known for a very long time to use xenon at relatively high pressure in order to increase the luminous efficacy in these lamps. For example, ii is stated in the relevant monograph, "The High-Pressure Sodium Lamp" of DeGroot/VanVliet (Philips Technical Library, Deventer, 1986) on pages 299 and 300, that an increase in the luminous efficacy by 10 to 15% can be obtained, if--in so-called Super Lamps--a cold filling pressure for xenon of 20 to 40 kPa (200 to 400 mb) is used instead of the standard conventional filling pressure of approximately 30 mb.
It is indicated simultaneously on page 299 that the luminous efficacy greatly decreases in high-pressure sodium discharge lamps with decreasing lamp power. Also, with elevated xenon pressure, it amounts to 85 lumens per watt (lm/W) at most for a 50 W lamp power, whereas a luminous efficacy of approximately 138 lm/W can be obtained for a 400 W lamp power.
A Hg-free high-pressure sodium lamp particularly suitable for so-called self-stabilizing operation is described in German Patent 2,600,351, and this has a sodium operating pressure p.sub.NaB of between 4 to 93 mb, a xenon operating pressure p.sub.Xe(hot) .gtoreq.800 mb and a pressure ratio p.sub.NaB /p.sub.Xe(hot) .ltoreq.1/20. Taking into consideration the usual factor 8 (German AS 2,814,882, column 2, center) for converting between xenon operating pressure and xenon cold filling pressure p.sub.XeK, there results a pressure ratio p.sub.XeK/p.sub.NaB .ltoreq.2.5. Under self-stabilizing operation, it is a desired goal to drive a high-pressure sodium lamp without a lamp ballast. A long decomposition time of the plasma formed from the filling gas is necessary for this mode of operation. In order to achieve this long decomposition time, as is known, a relatively high xenon pressure as well as a relatively large inner diameter of the discharge vessel is used (see also the above-mentioned pertinent monograph of DeGroot/VanVliet, pages 126 and 154). According to DeGroot/VanVliet, p. 155, the self-stabilizing operation of high-pressure sodium lamps has found no practical application due to problems in ignition and in sudden changes in the mains voltage.
The high-pressure sodium discharge lamp described as an example in German Patent 2,600,351 has a high power of 400 W and a very large inner diameter of 7.6 mm. The xenon cold filling pressure amounts to 260 mb and the pressure ratio p.sub.XeK /p.sub.NaB is approximately 3.5. Thus, a rather moderate luminous efficacy of only 110 lm/W is obtained with a high power of 400 W. A particularly high luminous efficacy is neither aimed at nor achieved in this publication in comparison to other high-pressure sodium lamps. According to FIG. 10.18 of DeGroot/VanVliet (p. 299), luminous efficacies of up to 138 lm/W can be obtained for 400 W powers. This dependence in principle of luminous efficacy on lamp power is shown for purposes of comparison as FIG. 3 (see below).
A Hg-free high-pressure sodium lamp without self-stabilization is described in German AS 2,814,882. A value between
1.25&lt;p.sub.XeK /p.sub.NaB &lt;6 with p.sub.NaB =150 to 500 mb PA1 1. A lower wall temperature of the discharge vessel can be achieved due to the smaller heat loses. This can be utilized, for example, for prolonging the service life. Alternatively, the discharge vessel can be reduced in size, so that the initially present will temperature is again achieved. Due to the higher power density, the luminous efficacy increases still further. PA1 1. For a sodium vapor pressure of 20 to 100 mb, the temperature of the discharge vessel at the coldest point (cold spot) amounts to only 840 to 950 K. This coldest spot always lies in the vicinity of the seal. Thus, the seal is typically approximately 150 K colder than in previously known lamps (see German AS [Examined] 2,814,882), for which reason, there is a reduction in lamp failures due to leaks in the region of the seal. PA1 2. The corrosion of the wall of the discharge vessel, which is due to sodium and which occurs preferentially in the center of the vessel, is reduced due to the low sodium partial pressure. In this way, there results an additional increase in service life.
is recommended for the xenon cold filling pressure p.sub.XeK relative to the sodium operating pressure (p.sub.NaB =sodium operating pressure). This value moreover agrees quite well with the value described in German Patent 2,600,351 for the pressure ratio p.sub.XeK /p.sub.NaB. However, a further increase of the xenon pressure above this upper limit is not recommended in German AS 2,814,882 (column 3, lines 41f), since the disadvantage of a more difficult ignition results, "without a corresponding increase in luminous efficacy". In the examples with low lamp powers of 70 and 100 W, p.sub.NaB =230 mb, the xenon cold filling pressure is approximately 500 mb. This corresponds to a pressure ratio p.sub.XeK /p.sub.NaB of approximately 2 to 2.5. Therefore, a luminous efficacy of 97 to 105 lm/W is obtained at 70 or 100 W. These values are plotted for comparison as the x's in FIG. 3 (see below).